Electrical energizing circuit for a piezoelectric element



June 27, 1967 M; CLICQUES 3,328,609

ELECTRICAL ENERGIZING CIRCUIT FOR A PIEZOELECTRIC ELEMENT Filed Oct. 20, 1964 United States Patent F 8 Claims. ci. 310-81) This invention relates to an arrangement for electrically energizing piezoelectric transducers and their electrostrictive and magnetostrictive analogs, and particularly to an energizing circuit for transducers of the type employed to generate ultrasonic vibrations for testing the homogeneity of structual elements, and more particularly of metallic elements.

It is the primary object of the invention to provide energizing apparatus for such transducers which is simpler and more compact than currently employed energizing devices.

Another object is the provision of an energizing circuit for transducers of the type described which can be operated with a power supply of relatively low voltage.

A general object is the provision of energizing apparatus for transducers which is readily portable.

According to one aspect of the invention, the electrical energizing arrangement for piezoelectric transducers and the like includes a transistor which is subject to avalanche breakdown, that is, having a characteristic collector current-voltage curve which has a portion of infinite slope adjacent another portion of negative slope. The piezoelectric transducer is arranged in such a manner that the sudden potential drop caused by the transistor when it operates in avalanche breakdown is at least partly applied to the terminals of the transducer, directly or indirectly.

Other features and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description of preferred embodiments when considered with the appended drawing in which:

FIG. 1 shows a characteristic voltage vs. current curve of a transistor employed in this invention;

FIG. 2 is a schematic of an energizing circuit for a piezoelectric transducer according to the invention;

FIGS. 3 to 5 are schematics of modifying details for the circuit of FIG. 2; and

FIG. 6 is a schematic showing another energizing circuit of the invention for a piezoelectric crystal.

Before discussing this invention in detail, it is pertinent to note that electrical energizing devices used heretofore for piezoelectric crystals which are intended to emit ultratherefore necessary, prior to this invention, to install a high-tension energizing apparatus separately from the transducer, and to connect the transducer to the energizing apparatus by a cable. This connection causes distortion of the transmitted signals and a loss of energy which significantly reduces the output efficiency of the energizing apparatus. The entire arrangement is complex, bulky, heavy, and costly.

It has been found that the shortcomings of the known devices can be overcome by utilizing the avalanche breakdown of a transistor, and by applying to the crystal to be energized all or at least a portion of the sudden voltage change caused by such breakdown of the transistor.

Many transistors are suitable for such operation, and particularly those known as mesa transistors.

3,323, Q Patented June 27, 1967 FIG. 1 shows the current I in the collector circuit of such a transistor as the function of the potential V applied to the collector and emitter of the transistor when the base-emitter potential has a value sufficient normally to block the transistor, this value being positive, zero, or slightly negative for a P-N-P transistor, and negative, zero, or slightly positive for an N-P-N transistor.

At a collector potential V the characteristic curve of the transistor has a slope a l /dV which is infinite (at the point A), and an adjacent portion of the curve, indicated by a broken line, has a negative slope. As soon as the applied collector potential V reaches the value V a very high current passes through the collector circuit while the collector potential drops abruptly to a limiting value V In many known types of transistors, the collector current during avalanche breakdown may amount to several amperes without damage to the transistor.

In the energizing devices of the invention, the abrupt potential drop at a transistor which is in a condition of avalanche breakdown is applied to a crystal employed as a transducer.

In the first embodiment of the invention shown in FIG. 2, the terminals 9 and 10 of the crystal transducer 1 are respectively connected to the grounded base lead 6 and the collector 2 of the transistor 3 which is of the N-P-N type. The collector is supplied with voltage from the positive terminal of a battery 4 through a resistor 5. The emitter 7 of the transistor 3 is grounded through a resistor 8.

The apparatus represented by the schematic of FIG. 2 operates as follows:

The transistor is initially blocked since its base is at ground potential. The battery 4 gradually charges the capacitor constituted by the crystal 1 through the resistor 5. As soon as the potential at the crystal terminals 9, 10 reaches the value V for the transistor 3, avalanche breakdown of the transistor causes discharge of the crystal charge, and the discharge current increases in magnitude as the potential between collector and emitter decreases,

crystal is extremely rapid, and the voltage at the terminals 9, ltl drops from V to V within a period which is of the order of magnitude of a nanosecond (10 second).

This abrupt potential drop initiates a series of vibrations in the ultrasonic frequency range in the crystal, and thus produces the desired mechanical efiect.

After discharge, the collector current again becomes very weak, and the transistor is blocked. Point B in FIG. 1 represents the condition of the transistor at this stage. A new cycle begins with the charging of the crystal 1 by the battery 4.

If it is desired that the crystal emit only brief bursts of ultrasonic oscillations, such as those employed in ultraow inspection, the vibrations of the crystal may be damped either by known mechanical means, or electrically by shunting the terminals 9, lit with a damping circuit.

Two damping circuits are diagrammatically illustrated n FIGS. 3 and 4 which only show the terminals 9 and 10 of the circuit of FIG. 2.

The damping circuit of FIG. 3 consists solely of a resistor 11 in series with a capacitor 12 whose capacitance is much smaller than that of the crystal 1 and offers only low impedance at the resonant frequency of the latter.

The circuit of FIG. 4 includes an inductance 13 arranged in series with the resistor 11' and the capacitor 12'.

If it is desired to employ the crystal 1 not only as a generator, but also as a receiver of ultrasonic oscillations, it is linked with an amplifier and a load, jointly indicated by numeral 14, by a coaxial cable 15 suitably terminated in its characteristic impedance at its output terminals by resistor 16. The resistor 16 is arranged in series with a capacitor 17 in order not to interfere with the charging of the crystal 1 by the battery '4. In this arrangement, the resistor 16 may function in the same manner as the damping resistor 11 in the circuit of FIG. 3. In the modified circuit illustrated in FIG. 6, the crystal 1' is conductively interposed between the emitter 7 of the transistor 3 and ground. A capacitor 18 is arranged between the collector 2 and ground. The battery 4 provides potential to the collector of the transistor 3, which is of the N-P-N type, through a resistor 5' and the coaxial line 15 which is terminated by resistor 16 and is suitable for transmitting electrical signals developed by the crystal 1 to the load 14 when the crystal operates as a receiver.

The base 6 of the transistor 1 is grounded through a variable resistor 19 which is negatively biased by a battery 20, whereby the return of the transistor to its blocked state is accelerated.

The advantage of the modified circuit of FIG. 6 over the circuit of FIG. 2 resides in the fact that the adapting resistor 16 may be arranged directly in parallel with the crystal 1 without the need for a capacitor as shown at 17 in FIG. 5 since the charging circuit of the capacitor 18 is separated from the crystal 1 when the transistor is in the non-conducting state. Moreover, the resistor 16 may simultaneously function as a damping resistor for the vibrations generated by the crystal. On the other hand, the circuit of FIG. 6 does not permit total use of the voltage drop from V to V in the collector circuit.

The portion of the sudden potential drop V V which is effectively applied to the terminals of the crystal 1' in the device of FIG. 6 may be calculated from the equation wherein C is the capacitance of the crystal 1, C is that of the capacitor 18, V is the potential at the terminals of the crystal 1 after avalanche breakdown of the transistor 3. The equation is derived from the fact that the charges before and after avalanche breakdown are equal, when disregarding the very small charge which flows through the resistor 16 during the brief period of avalanche breakdown:

The ratio V /(V V approaches unity when C is increased or C is reduced. It is therefore desirable to make the capacitance C of the capacitor 18 much larger than that of the crystal 1. When C=4C, four fifths of the abrupt voltage change V V are utilized.

Certain practical limitations are imposed on the magnitude of the capacitance C because of the fact that the time required for charging the capacitor may be unduly extended and that the discharges may be too infrequent for some applications of the transducer.

Instead of waiting for the collector potential to .reach a value V sufficient for avalanche breakdown during normal charging of the crystal 1, one may initiate each discharge at an earlier moment by transmitting synchronization pulses (negative in the instant device) generated by a pulse generator 22 to the base 6 of the transistor 3 through a decoupling capacitor 21.

It has been shown hereinabove with reference to FIG. 2 that a resistor 8 may be employed for limiting the current released by the transistor 3. The same object is achieved in the circuit of FIG. 6 by the insertion of the inductance 23 in the discharge circuit of the capacitor 18, either in the collector lead, as illustrated, or in the emitter lead. This arrangement has the following advantage:

If it is desired that the discontinuous emission of ultrasonic signals be repeated at brief intervals, it is necessary, for a given set of capacitance values, that either the resistance of the element 5 be reduced or the voltage of the battery 4 be increased. The first-mentioned change is simple, but the resistance of the resistor 5 should be sufiicient to ensure that the strength of the current which passes after avalanche breakdown be located below the point A in the characteristic curve of FIG. 1. If this would not otherwise be the case, the inductance 23 has a necessary corrective effect.

After the discharge has been initiated, it is continued through the inductance in the conventional manner of capacitor discharge in a circuit consisting of an inductance and a capacitor connected in series, until the potential reaches a value lower than V whereby the condition of the system is represented by a point adjacently to the left of the point B in FIG. 1.

Embodiments of the invention represented by the circuits of FIGS. 2 and 6 have been successfully operated for discontinuous emission of ultrasonic vibrations with a battery 4 of only volts and with a silicon mesa transistor 3 or 3 of N-P-N type commercially available as type 2N'706A (Texas Instrument Company).

The other circuit elements shown in the drawing had the following values:

Resistors:

5 330K 8 22 11 11 150 16 150 5' 330K 19 ohms 4700 Capacitors:

2 4700 12 4700 17 4700 18 22,000 21 picofarads 470 Inductances:

13 1 23 microhenry 1 The crystals 1, 1 each consisted of barium titanate, and had a capacitance of 10,000 picofarads, resonant at 4 megahertzs. The coaxial cable 15 was 15 meters long and had an impedance characteristic of 150 ohms. The amplifier 14 was constituted by a conventional band pass amplifier centered on the frequency of the crystal and having 1 MHz bandwith. The cable was feeding on its characteristic resistance. The pulse generator 22 was a 10-volt generator of positive pulses, having a pulse time of 0.05 microsecond. The voltages of batteries at 20 and 4 were respectively10 volts and 4 volts.

The energizing circuit of the invention requires little space and its weight can be so small that the entire apparatus becomes readily portable. It can be constructed at low cost. Because of the shortness of the excitation and the close spacing between crystal and transistor possible in the devices of the invention, the efficiency is excellent.

It should be understood, of course, that the foregoing disclosure relates only to preferred embodiments of the invention, and that it is intended to cover changes and modifications of the examples of the invention chosen herein for the purpose of the disclosure which do not constitute departures from the spirit and scope of the invention set forth in the appended claims.

What is claimed is:

1. Apparatus for periodic generation of ultrasonic energy comprising:

(a) a piezoelectric electromechanical transducer element;

(b) a transistor subject to avalanche breakdown;

(c) a source of electric potential;

((1) first circuit means connecting said transistor to said source for inducing avalanche breakdown of said transistor; and

(e) second circuit means connected to said transistor and adapted to be connected to a piezoelectric element for applying at least a portion of the potential drop occurring at said transistor during said breakdown to said piezoelectric element.

2. An arrangement as set forth in claim 1, wherein said transistor is of the N-P-N type, and said second circuit means are adapted for conductively interposing said piezoelectric element between the collector of said transistor and ground, the first circuit means including two resistors, said collector being connected to said source through one of said resistors, the emitter of said transistor being connected to ground through the other resistor, and the base of said transistor being directly connected to ground.

3. An arrangement as set forth in claim 1, wherein said transistor is of the type N-P-N, said second circuit means are adapted for conductively interposing said piezoelectric element between the emitter of said transistor and ground, said first circuit means including a resistor and a capacitor, the collector of said transistor being connected to said source through said resistor and being connected to ground through said capacitor.

4. An arrangement as set forth in claim 3, further comprising a piezoelectric element conductively interposed by said second circuit means between said emitter and ground, the capacitance of said capacitor being substantially greater than the capacitance of said piezoelectric element.

5. An arrangement as set forth in claim 4, including a damping resistor arranged in parallel circuit with said piezoelectric element.

6. An arrangement as set forth in claim 5, including coaxial cable means connected to said piezoelectric element and adapted to be connected to a load for trans mitting to said load electrical signals generated by said element, said damping resistor being connected to the load end of said cable means and having a resistance equal to the characteristic resistance thereof.

7. An arrangement as set forth in claim 3, further comprising a negatively polarized variable resistor, the base of said transistor being connected to ground by said variable resistor.

8. An arrangement as set forth in claim 3, wherein the collector of said transistor, the emitter of said transistor, and said capacitor are connected in series to constitute elements of a discharge circuit for said capacitor, said discharge circuit further including a series-connected inductance.

References Cited UNITED STATES PATENTS 2,562,450 7/ 1951 DeLano 3108 2,594,841 4/1952 Arndt 3108.1 2,692,337 10/1954 Hanson 331-116 2,825,813 3/1958 Sperling 331116 2,852,676 9/1958 Joy 3108.1 3,080,489 3/1963 White 30788.5 3,141,981 7/1964 Henebry 30788.5 3,200,350 8/1965 Sharp 331116 3,225,313 12/1965 ReXroad 3311l6 3,258,720 6/1966 Tartas 331116 FOREIGN PATENTS 814,185 6/ 1957 Great Britain.

MAX L. LEVY, Primary Examiner. J. D. MILLER, Assistant Examiner. 

1. APPARATUS FOR PERIODIC GENERATION OF ULTRASONIC ENERGY COMPRISING: (A) A PIEZOELECTRIC ELECTROMECHANICAL TRANSDUCER ELEMENT; (B) A TRANSISTOR SUBJECT TO AVALANCHE BREAKDOWN; (C) A SOURCE OF ELECTRIC POTENTIAL; (D) FIRST CIRCUIT MEANS CONNECTING SAID TRANSISTOR TO SAID SOURCE FOR INDUCING AVALANCHE BREAKDOWN OF SAID TRANSISTOR; AND (E) SECOND CIRCUIT MEANS CONNECTED TO SAID TRANSISTOR AND ADAPTED TO BE CONNECTED TO A PIEZOELECTRIC ELEMENT FOR APPLYING AT LEAST A PORTION OF THE POTENTIAL DROP OCCURRING AT SAID TRANSISTOR DURING SAID BREAKDOWN TO SAID PIEZOELECTRIC ELEMENT. 